Ceramic body, especially adapted for use as a spark plug insulator



Patented Feb. 10, 1942 OFFICE CERAMIC BODY, ESPECIALLY ADAPTED FOR use AS a seam: rnoomsom'roa I Albra n. Fessler and Karl Schwart zwalder, rum,

Mich., assignors to General Motors corpora-m 1' tion, Detroit, Mich, a corporation of Delaware No Drawing.

Application February 19, 1938,

Serial No. 191,400 Claims. (Cl. 106-46) This is a continuation in part'of our prior application Serial No. 48,902, filed November 8, 1935.

This invention has to do withceramic bodies especially adapted for use as insulators for spark plugs. Such insulators must be non-porous and must possess superior electrical resistance at high temperatures, good resistance to heat shock so that sudden changes in temperature will not produce failures, and must be strong mechanically.

We have found that the addition of zircon in substantial amounts to alumina, followed by firing the body at hightemperatures, produces a body having the above properties to an unusual degree.

To make the improved body alumina, which has preferably been calcined and then finely ground, is mixed with finely ground zircon. The raw materials should be of a high degree of purity, preferably approaching or equalling in purity the grades known as C. P., and should be substantially free from alkalies as these have the effect of reducing the dielectric strength of the insulator. I

Any suitable known methods may be employed in the purification of the raw materials. We have found the method of removing alkalies by treatment with boric acid as described and claimed in Patent No. 2,069,060 granted to Albra H. Fessler on January 26, 1937 to be very satis' factory.

Th raw materials should begthoroughly mixed. If the desired degree or subdivisionhas not been accomplished before mixing the materials should be subjected to further grinding. We have found it desirable to carry the grinding to a point where the major portion of the material is of a grain size between 2 /2 and 30 microns while the remainder is for the most part of but slightly larger grain size. While the grain size i not critical it is important that the material be of this general order of fineness in the manufacture of such products as'sparkplug insulators in order to secure the desireddense, strong non-porous bodies.

forms required to make a single insulator shape are then assembled and formed into the desired shape under heat and pressure, the pressures preferably ranging from 10,000 to 25,000 pounds per square inch and the temperature being sufficient to set the Bakelite binder, so that a durable Bakelite-bonded body is produced.

The formed body is then fired to a sufilciently high temperature, 1600-1750 0., to vitrify the mass, producing a dense, non-porous body. The

high firing temperatures completely eliminate the temporary binder.

We have tested insulators of many difierent compositions falling within a range extending from very small amounts of zircon with a preponderance of alumina to very small amounts of alumina with a preponderance of zircon.

- With compositions containing from 1 to 15% zircon the final fired structure consists essentially-of corundum, interspersed zirconia and zirconium aluminum silicate glass. It has been found upon careful petrographic examination that no zircon remains as such in the final fired body. These bodies are marked by the fact that the corundum crystals which form a large proportion of the composition are substantially tree from voids which would be present were the zircon eliminated. In addition, the zircon appears to have the effect of reducing the size and growth of the crystals. The crystals are now embedded in the glass which materially increases the strength of the body.

The zircon-alumina bodies are characterized by different physical properties as compared with an all alumina body. The apparent specific gravity has been increased. Thus in the case of bodies containing 2 /2% zircon the apparent specific gravity is 3.78 compared with 3.70 in the case of an all alumina insulator, and this is accompanied by a reduction in closed pore space from 7 to 3.61%.

The addition of the zircon also has the important advantage of materially reducing the firing temperature-that is, the temperature at which a dense, non-porous matured body is obtained.

Thus, while the firing temperature for alumina bodies is on the order of 1750 6., when 5% zircon is added the firing temperature is reduced to the neighborhood of 1675. 0., and when 10% zircon is added the firing temperature is on the order or 1625 C. This is important commercially because oi! the difilculty oi constructing and maintaining furnaces capable of operating at such high temperatures.

Spark plug insulators made of compositions containing up to 15% zircon are especially good in thermal shock resistance, thermal conductivity and resistance to electrical leakage at high temperatures, as well as to high frequency current and, in general, they are well adapted not only to give care-free service in present day automobile engines, but also in future designs in which, because of higher compressions or other changes in engine construction, the burden on the insulator may be far greater than at present.

In commercial manufacture or spark plug insulators in accordance with this invention, we prefer to employ on the order of from 2 to 5% zircon, thereby obtaining desirable physical properties to a degree to satisfy practical requirements at aminimum of cost, and with the advantage of comparatively easily obtainable processing temperatures.

According to Dana's Textbook on Mineralogy,

fourth edition (1932), page 610; zircon is found body. This group of bodies when employed asby chemical analysis to have an oxide content of approximately 67.2% zirconium oxide and 32.8% silica. Accordingly bodies made accordlng to this invention from batches consisting of approximately 1 to zircon and from 85 to 99% aluminum oxide would show upon chemical gether with a ceramic flux.

analysis approximately the following oxide content:

Per cent Aluminum oxide (A1203) 85 to 99 Zirconium oxide (ZIOz) .6! to 10 Silica (S102) .35 to 5 With compositions containing 15% or more of zircon, insulators are obtained having a final structure consisting essentially of corundum,

mullite, zirconia and glass. With larger amounts of zircon the percentage of corundum decreases, while the percentages of the other constituents increase, resulting in a somewhat less dense spark plug insulators will be found to possess the above recited properties to a considerable degree.

modify the characteristics or the body. For example, small proportions of flux, such as magnesium silicate in the form of talc may be added to increase the electrical resistance at high temperatures or for other purposes. However, any such additions should not fundamentally change the structure'of the final body.

Bodies as above described with the addition of magnesium flux, e. g., talc. are specifically covered in our copending application S. N. 331,211,

filed April 23, 1940, said application being a continuation-in-part of the present application.

We claim:

1. Ceramic material in the form of a dense. non-porous, sintered mass showing upon chemical analysis a content of approximately 85 to 99% aluminum oxide, from .67 to 10% zirconium oxide and from .33 to 5% silicon dioxide, said material being substantially free from zircon.

2. Ceramic material in the form 01' a dense, non-porous sintered mass showing upon chemical analysis a content of approximately from .67 to 10% zirconium oxide, at least approximately .33 to 5% silicon dioxide, the-remainder consisting of a preponderance of aluminum oxide to- 3. A spark plug insulator in the form of a dense, non-porous mass characterized by high electrical resistance at both low and elevated temperatures, high mechanical strength, good resistance to heat shock and good thermal conductivity, showing upon chemical analysis a content of approximately 85 to 99% aluminum oxide, from .67 to 10% zirconium oxide, and from .33 to 5% silicon dioxide, said material being substantially free from zircon.

4. A spark plug insulator in the form of a dense, non-porous mass characterized by high electric-alresistance at both low and elevated temperatures, high mechanical strength, good resistance to heat shock and good thermal con- Even when alumina is totally eliminated a de- I ductivity, showing upon chemical analysis approximately from .67 to 10% zirconium oxide, at least approximately .33 to 5% silicon dioxide, the remainder consisting of? a preponderance of aluminum oxide together with a ceramic flux.

5. A spark plug insulator in the form or a dense, non-porous mass characterized by high electrical resistance at both low. and elevated temperatures, high mechanical strength, good resistance to heat shock and good thermal conductivity, showing upon chemical analysis approximately from .67 to 10% zirconium oxide, at

. ductivity, showing upon chemical analysis a conin the form of its constituents, zirconia and silica. Nor is it essential that the ratio of zircon'ia to silica existing in zircon be maintained where these materials are added insteadof zircon. Thus we may use a slight excess oi! either zirconia or silica without adversely afi'ecting the final body and such modifications we regard as within the scope of our invention. Other materials may be added in small amounts to slightly tent or approximately to 97%% aluminum oxide, 1.68 to 3.36% zirconium oxide and from .82 to 1.64% silica, said insulator being substantially free from zircon.

7. A spark plug insulator in the form or a dense, non-porous mass characterized by high electrical resistance at both low and elevated temperatures, high mechanical strength, good resistance to heat shock and good thermal conductivity, showing upon chemical analysis a con-- tent of approximately 1.63 to 3.36% zirconium oxide, from .82 to 1.64% silica. the remainder consisting of aluminum oxide, together with a small proportion of ceramic flux.

8. The process or making ceramic bodies which consists in preparing a finely pulverized mixture of from 1 to 15% zircon, the balance consisting of a preponderance 01' aluminum oxide together with a ceramic flux, shaping the body therefrom and firing the body at temperatures sufficiently while retaining substantially the original shape, the resultant body consisting of corundum together with interspersed zirconium oxide crystals in a glass matrix and being substantially free from zircon.

10. The process of making ceramic bodies which consists in preparing a finely pulverized mixture consisting predominantly of aluminum high to decompose the zircon and sinter the body to sinter the body into a dense, non-porous mass oxide together with a zirconium compound and alkaline earth flux, said mixture being in part siliceous, shaping the body therefrom and firing the body at temperatures sufliciently high to sinter the body into a dense, non-porous mass while retaining substantially the original shape,

the resultant body consisting of a preponderance of corundum together with interspersed zirconiuni oxide crystals in a glass matrix and being substantially free from zircon. I

ALBRA H. FESSLER. KARL SCHWARTZWALDER. 

